The main cause of tuberculosis (TB) in humans is the bacterium Mycobacterium tuberculosis. Most cases of the disease involve the lungs. The bacterium can enter cells and remain inactive in a latent (dormant) state. However, the bacterium can become active to produce symptoms of the disease. In a person with active TB, the pathogen can be present in airborne droplets that are exhaled. Generally, a healthy person who inhales these droplets has effective defence mechanisms in the gas exchange system to prevent infection.

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2018). The mark or mark type is shown in brackets.

1. [1] bone marrow ;
2. [3] max three from: 1 detection / recognition ; e.g. detects (named type of) pathogen recognises, (foreign) antigens / antibodies complexed to antigens has receptors (for antigens) 2 engulfs / envelops, pathogen / bacterium / AW ; A phagocytosis occurs A endocytosis occurs A pseudopodia form 3 forms, vacuole / vesicle / phagosome; ref. to hydrolytic / digestive, enzymes ; 4 ref. to lysosome fusion ; 5 A named examples A hydrolases 6 ref. to antigen presentation; 7 AVP; e.g. (response is) non-specific / innate
3. [1] any one valid suggestion from: produces inhibitors for / deactivates, lysosomal enzymes ; escapes out of phagosome; forms resistant spore / is resistant to digestive enzymes ; AVP; e.g. suggestion of macrophage malfunction
4. [3] any three from: 1 production of mucus by, mucous glands / goblet cells ; 2 sticky / AW, mucus or mucus traps, pathogens / bacteria / microorganisms ; 3 mucus acts as a barrier (to prevent entry); 4 mucus increases distance to reach cells ; 5 cilia on ciliated epithelial cells ; A ciliated epithelium 6 cilia, waft / move, mucus / AW ; 7 idea that (contaminated) mucus is moved, away from alveoli / away from lung tissue / towards back of mouth / AW ;
5. [1] any one suggestion from: blood / plasma / circulatory system ; lymph / lymph system ; within, neutrophils / macrophages / phagocytes ; A white blood cells / leucocytes
6. [1] any one from: from infected, cows / cattle ; eating contaminated, meat / beef (from infected cattle); drinking, raw / unpasteurised, milk (from infected cows);
7. [2] any two from: increases chances of, killing / AW, all bacteria; I will kill all bacteria (unqualified) A will kill all bacteria if bacteria are, susceptible / not resistant if bacteria are resistant to one antibiotic, then still susceptible to other antibiotics / AW ; reduces chance of mutations arising / (if bacteria susceptible) mutations unlikely to occur against all antibiotics ; AVP; e.g. antibiotic can be stopped because of side effects idea that more effective because different antibiotics will work on different targets (in the bacteria)
8. [2] any two from: overall trend for both is increase ; rate of increase in cases of MDR-TB greater than rate of increase in cases of TB / AW / ora ; ref. to fluctuations / detail of fluctuations / periods of decrease ;
9. [3] any three from: overall increase in TB cases: 1 ref. to problems with people living in, poorly ventilated accommodation / close proximity to each other/AW; 2 vaccination programmes not able to prevent increase / herd immunity insufficient / AW ; decrease/numbers fairly constant: 3 ref. to (some) success of, prevention / treatment programme ; A described examples increase in MDR-TB: 4 people fail to finish course of treatment for normal TB / AW ; 5 ref. to ease of transmission of TB means easy to transmit MDR-TB ; decrease in 2014 for MDR-TB: 6 suggestion for decrease ; e.g. better, surveillance / cooperation, in completion of drug therapy more successful antibiotics used people better educated about preventing spread for either increase in TB cases or increase in MDR-TB: 7 qualified ref. to link between, HIV / HIV/AIDS, and TB ; 8 less money available for, longer drug treatment/vaccination, by, health authorities / governments ; 9 AVP; e.g. balance between new cases and successfully treated cases means problem still exists in prevention ref. to changes in population size ref. to improvements in data collection ref. to immigration